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Theropoda
Theropoda is a clade of bipedal, mostly carnivorous saurischian dinosaurs. It includes modern birds, as well as some of the most famous dinosaurs of all, like Tyrannosaurus, Allosaurus, Velociraptor, ''and ''Spinosaurus. The largest theropods'', especially ''Tyrannosaurus, are a common feature in popular culture, often as an antagonist. Theropods have been present since the late Triassic, as small, nimble predators living in the shadow of enormous rauisuchians. In the Jurassic and Cretaceous periods, however, the carnosaurs, coelurosaurs, and megalosaurs evolved to become the largest terrestrial predators the world has seen, and they live on as birds, the amniote clade with the most species today. Characteristics All known theropods are bipedal, and most are carnivorous, with a few exceptions such as the therizinosaurs. For a long time, theropods, along with all dinosaurs, were considered to be lumbering, sluggish, quintessentially reptilian beasts, scientists have recently found a variety of evidence for feathers and proto-feathers in many groups of theropod. 35 different species have been discovered with clear evidence of feathers, all of them coelurosaurs. Instead of feathers, some species, like the horned Carnotaurus, have bony osteoderms as an integument. The forelimbs are shortened in some groups such as the tyrannosaurids and abelisaurids, as they were not especially important to them, but in the piscivorous and possibly amphibious spinosaurids, the arms are elongated to catch fish more efficiently. Although their hands are often portrayed as highly mobile, theropods could not pronate their hands like primates today. The theropod hand is highly varied among different clades, including both the amount of digits and the flexibility of the hand. Evolution Some of the first known dinosaurs are theropods, like the large, carnivorous Herrerasaurids of Argentina and the omnivorous Eoraptor. In the Triassic period, though, theropods were not the apex predators. They could not take down the largest herbivores, like the armored aetosaurs and some large synapsids, and they could not stand up to the largest predators, including the aforementioned rauisuchians, which grew up to 10 meters. The first readily apparent theropods in the Triassic were the coelophysoids, including the famous namesake Coelophysis, which fed on small reptiles. The first truly large theropods appeared in the Jurassic, as they radiated into many different forms to fill the ecological niches left open by the Triassic-Jurassic extinction, with small insectivores, opportunistic scavengers, and massive apex predators. Allosaurus is one of the best known dinosaurs, and its possible synonym Epanterias reached 12 meters. Coelurosaurs, though, were the ones who filled the most niches, with the powerful tyrannosaurs, the swift dromeaosaurs, the intelligent troodontids, the bizarre therizinosaurs, and many more. Another, more divergent clade, the Megalosauria, gave rise to the enigmatic sail-backed spinosaurids, including the largest theropod of all - Spinosaurus. 'Classification' The most primitive known theropods are, obviously, the earliest ones known, including the aforementioned Herrerasaurids and Eoraptor. All of the others form the clade Neotheropoda, including the coelophysoids, dilophosaurids, and everything else, the Averostra. Nested within Averostra are the Ceratosauria and the Tetanurae. The Tetanurae contain the Orionides and several other genera. The Orionides comprise the megalosaurs and the Avetheropoda. Carnosauria and Coelurosauria are nested within Avetheropoda.